We wish to understand the regulated expression of the globin and immunoglobulin genes of man and the mouse. In order to do this we have cloned the human and murine kappa and lambda light chain and heavy genes; the alpha and beta globin genes and pseudogenes of the mouse were also cloned. Detailed structural studies have been carried out that allow us to make certain critical inferences about mechanisms and nucleotide sequences that are involved in the transcriptional activation, somatic rearrangement and evolution of some of these genetic sequences. In particular, we have defined a new class of genetic elements that appears to have arisen via RNA intermediates. We have established features of the human lambda and IgE loci and chromosomally mapped a number of the human Ig genes to bands involved in characteristic translocations. We have also detailed the structural requirements for globin gene promoter activity.